The Field of the Invention
The present invention relates to a method for producing lead frame devices for integrated circuits and in particular to lead frames having a metal-plastic composite structure.
There are a number of well known methods for making lead frames for integrated circuit devices, such as the method and resulting product described in U.S. Pat. No. 3,440,027. The method disclosed comprises bonding a metal layer to a plastic substrate, coating the metal with a photo resist material, imaging and developing the desired lead frame pattern on the photo resist, and subjecting the prepared strip to an etching process to etch away the excess metal portions. While this method has been demonstrated as being feasible, there are certain problems which are inherent therewith including the fact that stress is built up in the plastic laminate during subsequent processing, handling and bonding operations which can result in misplaced leads and/or defective bonds. This method also requires the use of a substantial amount of fabricated plastic film, which adds to the cost and requires the additional processing steps of slitting, coating, laminating, etc., in order to utilize the final packaging method. Further, only the IC chip bonding schemes which can be utilized are those which can be carried out at lower temperatures compatible with the plastic substrate.
Improvements over the above-discussed patent can be found in U.S. Pat. Nos. 3,686,991, and 3,763,404 in which the lead portions of a lead frame pattern are formed extending out over central apertures in the plastic substrate in a cantilever fashion in what is generally referred to as a beam-over-hole configuration. This structural arrangement can result in a reduction in the stresses applied to the various leads when they are subjected to a subsequent bonding step and allows for direct metal to metal heating for higher temperature bonding operation. However, the cantilever beams extending a substantial distance over the hole are readily deformed and misaligned during processing and shipping so that there can be a significant reject factor driving up the cost of such devices.